The Master Handbook of

Acoustics 3rd Edition

F. Alton Everest

TAB Books Division of McGraw-Hill, Inc.

New York San Francisco Washington, D.C. Auckland Bogota Caracas Lisbon London Madrid Mexico City Milan

Montreal New Delhi San Juan Singapore Sydney Tokyo Toronto

Contents Epigraph xvii

Introduction xix

1 Fundamentals of sound 1 The simple sinusoid 2 Sine-wave language 3

Propagation of sound 4 The dance of the particles 4 How a sound wave is propagated 6 Sound in free space 7

Wavelength and frequency 8 Complex waves 9

Harmonics 9 Phase 10 Partiais 12 Octaves 14

The concept of spectrum 14 Electrical, mechanical, and acoustical analogs 17

2 Sound levels and the decibel 19 Ratios vs. differences 19 Handling numbers 20 Logarithms 21 Decibels 21 Reference levels 22 Logarithmic and exponential forms compared 24

Acoustic power 25 Using decibels 26

Example: Sound-pressure level 27 Example: Loudspeaker SPL 27 Example: Microphone specifications 28 Example: Line amplifier 28 Example: General-purpose amplifier 28 Example: Concert hall 29 Example: Combining decibels 29

Ratios and octaves 30 Measuring sound-pressure level 31

3 The ear and the perception of sound 33 Sensitivity of the ear 33 A primer of ear anatomy 34

The pinna: Directional encoder of sound 34 Directional cues: An experiment 34 The ear canal 35 The middle ear 36 The inner ear 39 Stereocilia 39

Loudness vs. frequency 40 Loudness control 41 Area of audibility 42 Loudness vs. sound-pressure level 44 Loudness and bandwidth 45 Loudness of impulse 47 Audibility of loudness changes 49 Pitch vs. frequency 49

An experiment 50 Timbre vs. spectrum 51 Localization of sound sources 51 Binaural localization 54

Aural harmorücs: Experiment #1 55 Aural harmorücs: Experiment #2 55 The missing fundamental 55

The ear as an analyzer 56 The ear as a measuring instrument 56

An auditory analyzer: An experiment 56 Meters vs. the ear 57 The precedence effect 58 Perception of reflected sound 60 Occupational and recreational deafness 61 Summary 63

4 Sound waves in the free field 67 Free sound field: Definition 67 Sound divergence 68

Examples: Free-field sound divergence 69 Inverse Square in enclosed Spaces 70 Hemispherical propagation 71

5 Speech, music, and noise 73 The voice System 73

Artificial larynx 73 Sound spectrograph 74 Sound sources for speech 74 Vocal tract molding of speech 76 Formation of voiced sounds 77 Formation of unvoiced sounds 78 Putting it all together 78 Synthesized speech 79 Digital speech synthesis 79 Directionality of speech 80

Music 82 Wind instruments 83 Nonharmonic overtones 83 Dynamic ränge of speech and music 84

Power in speech and music 84 Frequency ränge of speech and music 85 Future dynamic-range requirements 85 Auditory area 86 Noise 88

Noise—the good kind 89 Random noise 89 White and pink noise 90

Signal distortion 93 Harmonie distortion 93

6 Analog and digital signal processing 99 Resonance 99 Füters 102

Active filters 102 Analog vs. digital filters 103 Digitization 103 Quantization 104 Digital füters 104

Application of digital signal processing (DSP) 105 Application of DSP to room equalization 106

Reverberation 107 Reverberation and normal modes 108 Growth of sound in a room 109 Decay of sound in a room 111 Idealized growth and decay of sound 111 Reverberation time 111 Measuring reverberation time 113 Impulse sound sources 113 Steady-state sources 114 Equipment 115 Measurement procedure 116

Analysis of decay traces 116 Mode decay variations 117

Writing speed 118 Frequency effect 119

Reverberation time Variation with position 119 Acoustically coupled spaces 120 Electroacoustically coupled spaces 121

Decay rate 121 Eliminating decay fluctuations 121

Influence of reverberation on speech 122 Influence of reverberation on music 123 Optimum reverberation time 123

Bass rise of reverberation time 126 Living room reverberation time 127

Artificial reverberation: The past 128 Artificial reverberation: The future 128 Arrival time gap 130 The Sabine equation 130

Reverberation calculation: Example 1 132 Reverberation calculation: Example 2 132

Reverberant field 133

Control of interfering noise 137 Noise sources and some Solutions 137

Airborne noise 139 Noise carried by structure 139 Noise transmitted by diaphragm action 139 Sound-insulating walls 139 Porous materials 140

Sound transmission Classification (STC) 141 Comparison of wall structures 142 Double Windows 145 Sound-insulating doors 147

Noise and room resonances 149 Active noise control 149

9 Absorption of sound 151 Dissipation of sound energy 151 Evaluation of sound absorption 152 Reverberation Chamber method 153 Impedance tube method 154 Tone-burst method 155 Mounting of absorbents 157 Mid/high frequency absorption by porosity 158

Glass fiber: Building insulation 158 Glass fiber: Boards 159 Acoustical tile 159

Effect of thickness of absorbent 159 Effect of airspace behind absorbent 161 Effect of density of absorbent 161 Open-cell foams 161 Drapes as sound absorbers 163 Carpet as sound absorber 167

Effect of carpet type on absorbance 167 Effect of carpet underlay on absorbance 167 Carpet absorption coefficients 168

Sound absorption by people 168 Absorption of sound in air 170 Low-frequency absorption by resonance 170 Diaphragmatic absorbers 172 Polycylindrical absorbers 175 Poly construction 178 Membrane absorbers 179 Helmholtz resonators 181 Perforated panel absorbers 183 Slat absorbers 188 Placement of materials 188

Reverberation time of Helmholtz resonators 188 Taming room modes 189 Increasing reverberation time 190 Modules 190

10 Reflection of Sound 193 Reflections from flat surfaces 193 Doubling of pressure at reflection 195 Reflections from convex surfaces 195 Reflections from concave surfaces 195 Reflections from parabolic surfaces 196 Reflections inside a cylinder 196 Standing waves 197 Reflection of sound from impedance irregularities 198 The corner reflector 199

Echo-sounding 201 Perceptive effects of reflections 201

11 Diffraction of sound 203 Rectilinear propagation 203 Diffraction and wavelength 204 Diffraction of sound by large and small apertures Diffraction of sound by obstacles 206 Diffraction of sound by a slit 208 Diffraction by the zone plate 208 Diffraction around the human head 208 Diffraction by loudspeaker cabinet edges 211 Diffraction by various objects 212

12 Refraction of sound 215 Refraction of sound 215

Refraction of sound in solids 216 Refraction of sound in the atmosphere 217 Refraction of sound in the ocean 220 Refraction of sound in enclosed spaces 221

13 Diffusion of sound 223 The perfectly diffuse sound field 223 Evaluating diffusion in a room 224

Steady-state measurements 224 Decay beats 225 Exponential decay 225 Spatial uniformity of reverberation time 227 Decay shapes 228 Microphone directivity 229 Room shape 229 Splaying room surfaces 234

Nonrectangular rooms 234 Geometrical irregularities 235 Absorbent in patches 235 Concave surfaces 238 Convex surfaces: The poly 238 Plane surfaces 240

14 The Schroeder diffusor 241 Schroeder's first acoustic diffusor 242 Maximum-length sequences 243 Reflection phase-grating diffusors 244 Quadratic-residue diffusors 245 Primitive-root diffusors 246 Quadratic-residue applications 247

Performance of diffraction-grating diffusors 248 Expansion of the QRD line 252 Solving flutter problems 253 Application of fractals 255 Diffusion in three dimensions 256 Acoustic concrete blocks 257 Measuring diffusion efficiency 258

Comparison of gratings with conventional approaches 260

15 Modal resonances in enclosed Spaces 265 Resonance in a pipe 265 Bathroom acoustics 267 Reflections indoors 268 Two-wall resonance 269 Waves vs. rays 270 Frequency regions 271 Dividing the audio spectrum 272 Wave acoustics 273

Mode calculations—An example 274 Experimental verification 276 Mode identification 277 Modedecay 278 Mode bandwidth 281 Mode pressure plots 283 Modal density 287 Mode spacing and coloration 287 Experiments with colorations 288 Simplified axial mode analysis 288 The Bonello criterion 291 Controlling problem modes 291 Mode summary 292

16 Reflections in enclosed Spaces 295 Law of the first wavefront 295 Mean free path 295

The effect of Single reflections 297 Perception of sound reflections 297 Perception of spaciousness 298 Image changes 299 Discrete echoes 299 Effect of angle of incidence on audibility of reflection 299 Effect of signal type of audibility of reflection 299 Effect of spectrum on audibility of reflection 299 Using reflection data 300

Large Spaces 300 Echoes 300

Spaciousness 301

17 Comb-filter effects 303 What is a comb filter? 303 Superposition of sound 304 Tonal Signals and comb filters 305

Combing of music and speech Signals 306 Combing of direct and reflected sound 306

Comb filters and critical bands 311 Comb filters in Stereo listening 311 Coloration and spaciousness 312 Combing in stereo microphone pickups 313 Audibility of comb-filter effects 313

Comb filters in practice 313 Estimating comb-filter response 317

18 Quiet air for the studio 321 Selection of noise criterion 321 Fan noise 323 ASHRAE 324 Machinery noise 325 Air velocity 325 Effect of terminal fittings 326 "Natural" attenuation 326 Duct lining 328 Plenum silencers 329 Packaged attenuators 330 Reactive silencers 330 Resonator silencer 331 Duct location 332 Some practical suggestions 333

19 Acoustics of the listening room 335 The acoustical link 335 Peculiarities of small-room acoustics 336

Room size 337 Room proportions 337 Reverberation time 338

The Nippon-Gakki experiments 338 The listening room: Low frequencies 338

Control of modal resonances 340 Bass traps for the listening room 342 Modal colorations 343 The listening room: The mid-high frequencies 343 Identification and treatment of reflection points 345 Lateral reflections: Control of spaciousness 346

20 Acoustics of the small recording studio 349 Acoustical characteristics of a studio .550 Reverberation 351 Studio design 352 Studio volume 352 Room proportions 354 Reverberation time 354 Diffusion 355 Noise 355 Studio design procedure 356 Some studio features 356 Elements common to all Studios 360

21 Acoustics of the control room 361 The initial time-delay gap 361 The live end 363 Specular reflections vs. diffusion 364 Low-frequency resonances in the control room 365 Initial time-delay gaps in practice 367 Managing reflections 367 The reflection-free-zone control room 369 Control-room frequency ränge 369 Outer shell of the control room 371 Inner shell of the control room 372 Representative control rooms 372 Some European designs 372 Consultants 377

22 Acoustics for multitrack recording 383 Flexibility 384 Advantages of multitrack 385 Disadvantages of multitrack 355 Achieving track Separation 387 Studio acoustics 387

Distance between artists 387 Microphone management 387 Barriers for Separation 388 Electronic Separation 388 Electronic instruments and Separation 388

The future of multichannel 388 Automation 389

23 Audio/video tech room and voice-over recording 391 Selection of space: External factors 391 Selection of space: Internal factors 392 Work space treatment 392

Audio/video work place example 392 Appraisal of room resonances 392

Control of room resonances 394 Treatment of work place 394

Calculations 394 The voice-over booth 396 Dead-end live-end voice studio 396 Voice-over booths 396 The Quick Sound Field™ 397

24 Adjustable acoustics 401 Draperies 401 Adjustable panels: Absorption 402 Adjustable panels: The Abffusor™ 402 Hinged panels 405 Louvered panels 406 Variable resonant devices 407 Rotating elements 408 Portable units: The Tube Trap™ 410 Portable units: The Korner Killer™ 414

25 Acoustical measurements 415 The microprocessor and microcomputer 415 The three measurement domains 416 Anechoic measurements 416 Steady state vs. transient measurements 416 Signals for acoustical tests 417 The tone burst as a test signal 417 Tone-burst measurements 418 Origins of time-delay spectrometry (TDS) 421

How TDS works 421 Putting TDS to work 422

The Fast Fourier Transform (FFT) 423 The Techron TEF™ Computer 423

Using the Techron TEF™ 424 Examples of Techron TEF™ output 426

The Techron TEF™ 20 system 427 Sound Lab™ Software 429 Sound Lab™ RTA-PC Software 429 AcoustaEQ™ Software 429 Speaker test—PC Software 430 Future Software 430 The TEF 20HI™ option 430

Appendix 433

Glossary 437

Index 447